This invention relates to a polyolefin resin composition filled with an inorganic filler in a specific manner to thereby improve the thermal resistance, stiffness, dimensional stability, and other properties thereof.
Extensive attempts have been made to improve thermal resistance, stiffness, dimensional stability etc., of polyolefin resins, especially crystalline polyolefin resins by filling them with a variety of inorganic fillers. Examples of such polyolefin resins are low-medium-or high-density polyethylene, isotactic polypropylene, crystalline ethylene-propylene copolymer, crystalline copolymers of a main component of ethylene and/or propylene and other ethylenically unsaturated monomer(s). Filling with inorganic fillers having relatively large aspect ratios such as fibrous, needle-like or lamellate fillers is effective. Especially, filling with glass fiber increases thermal resistance, stiffness, etc. of the resulting composition.
On the other hand, resin compositions filled with fibrous or lamellate fillers tend to cause poor appearance, decrease in impact strength, etc. of the resulting molded products. In order to eliminate such defects, several methods have been proposed, examples of which are: control of the fiber diameter of glass fiber (Japanese Laid-open patent application No. 226041/1984); control of particle size of talc (Japanese Laid-open patent application No. 222132/1983); and combined use of talc having a specified specific surface area and precipitated calcium carbonate (Japanese Laid-open patent application No. 16042/1982).
However, the materials obtained according to the above mentioned conventional methods in which the compositions are even filled with inorganic fillers exhibit marked decrease in stiffness at a temperature near the crystalline melting point of the matrix-forming polyolefin resin, for example, at a temperature over about 160.degree. C. in the case of isotactic polypropylene and over about 120.degree. C. in the case of high-density polyethylene. Thus, the utilization of such resin materials in the fields requiring higher stiffness, especially stiffness at high temperature, has been restricted.